Stockholders' Equity - Pre-2017 Non-Qualified Stock Options, Additional Information (Details) - Pre-2017 Non-Qualified Stock Options - USD ($) $ in Thousands	6 Months Ended
Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Options outstanding (in shares)	2,700,500		2,885,500
Options granted (in shares)	0
Options to purchase common stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation	$ 115	$ 570
Compensation cost not yet recognized	$ 74

Amount of cost not yet recognized for nonvested award under share-based payment arrangement.

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